Btrfs: don't map extent buffer if path->skip_locking is set

Arne's scrub stuff exposed a problem with mapping the extent buffer in
reada_for_search.  He searches the commit root with multiple threads and with
skip_locking set, so we can race and overwrite node->map_token since node isn't
locked.  So fix this so that we only map the extent buffer if we don't already
have a map_token and skip_locking isn't set.  Without this patch scrub would
panic almost immediately, with the patch it doesn't panic anymore.  Thanks,

Reported-by: Arne Jansen <sensille@gmx.net>
Signed-off-by: Josef Bacik <josef@redhat.com>

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index d840893..2e66786 100644 (file)
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -1228,6 +1228,7 @@ static void reada_for_search(struct btrfs_root *root,
        u32 nr;
        u32 blocksize;
        u32 nscan = 0;
+       bool map = true;
 
        if (level != 1)
                return;
@@ -1249,8 +1250,11 @@ static void reada_for_search(struct btrfs_root *root,
 
        nritems = btrfs_header_nritems(node);
        nr = slot;
+       if (node->map_token || path->skip_locking)
+               map = false;
+
        while (1) {
-               if (!node->map_token) {
+               if (map && !node->map_token) {
                        unsigned long offset = btrfs_node_key_ptr_offset(nr);
                        map_private_extent_buffer(node, offset,
                                                  sizeof(struct btrfs_key_ptr),
@@ -1277,7 +1281,7 @@ static void reada_for_search(struct btrfs_root *root,
                if ((search <= target && target - search <= 65536) ||
                    (search > target && search - target <= 65536)) {
                        gen = btrfs_node_ptr_generation(node, nr);
-                       if (node->map_token) {
+                       if (map && node->map_token) {
                                unmap_extent_buffer(node, node->map_token,
                                                    KM_USER1);
                                node->map_token = NULL;
@@ -1289,7 +1293,7 @@ static void reada_for_search(struct btrfs_root *root,
                if ((nread > 65536 || nscan > 32))
                        break;
        }
-       if (node->map_token) {
+       if (map && node->map_token) {
                unmap_extent_buffer(node, node->map_token, KM_USER1);
                node->map_token = NULL;
        }